The invention relates to a non-destructive method of testing a weld comprising a plurality of strands of electrical conductors joined together into a bundle by ultrasonic welding. Furthermore the invention relates to a testing apparatus for implementing the cited test method and to an ultrasonic welding machine having such an apparatus.
xe2x80x9cWeldxe2x80x9d in this context covers any welded connection comprising, more particularly, the following properties. Firstly, two or more stripped strands are joined together by an ultrasonic welding method. It is understood that a strand of an electrical conductor or cable comprises a plurality of single wires. The single wires are designed to conduct an electrical current and are normally made of copper or some other material as generally appreciated in the field concerned. Ultrasonic welding is characterized by each of the single wires to be joined together being heated up by vibration, disrupting the surfaces of the single wires, smoothing out surface irregularities and thus achieving a weld. The stripped strands are typically arranged parallel to each other and then welded to each other cylindrically by some kind of section over a predetermined length. More particularly, jointing is also promoted by the single wires being crimped together.
Welds of the aforementioned kind need to be fabricated very often in producing wiring harnesses. These can be wiring harnesses for motor vehicles, commercial vehicles as well as for components for aircraft and marine applications. In producing a wiring harness on a so-called makeup board a weld is produced directly in the wiring harness by an ultrasonic welder in creating an electrical connection between several electrical conductors.
If inspection were to find a weld to be insufficient or unsatisfactory, i.e. failing to produce a satisfactory electrical connection or lacking in mechanical (bonding) strength when the wiring harness has already been installed, the entire damaged wiring harness would need removing as a whole and to be replaced new. This is prohibitively costly and time-consuming. Accordingly, it is especially in just-in-time production in automotive engineering not permitting any delay, that it is unacceptable for wiring harnesses to be installed with faulty or NO GO (defective) welds.
This is why attempts have been made to develop test methods which permit testing the welds (welded connections) produced prior to the wiring harnesses being installed. However, to date only tests could be implemented which involve destroying the weld. For this purpose peel or bending tests have been developed, pull tests implemented and micrographs analyzed. For one thing, this is unacceptable economically since this also involves destroying GO welds at great expense. For another, this test method can be carried out naturally only on a sampling basis, thus risking NO GO wiring harnesses being installed.
It is due to this that attempts have already been made to check the welding properties already in the ultrasonic welding machine. For this purpose an internal quality control, a so-called inspector was installed in the system for monitoring a plurality of trouble sources such as for example oxydized conductors, grease or oil inclusions in the weld, copper quality. In addition the weld is gauged to establish its degree of density from the ratio of height to width. However, this test method is also complicated and inaccurate.
Attempts have also been made to obtain an indication as to the quality of the weld already during welding by testing its ring in sound. This involves simulating various factors having a negative influence on the weld. But this method too, failed to lead to any positive result. Also investigated was the possibility of a ultrasound or eddy current test since these methods are already put to use in checking for inclusions in castings or for welding seam quality assessment. However, this composition bears no comparison with those in a weld of the aforementioned kind since unlike welded strands a solid structure is involved, thus making it relatively easy to analyze the quality.
The technical problem on which the invention is based involves comprising a method of non-destructively testing a weld of the aforementioned kind which is simple and reliable in application, also in accompanying the process to permit obtaining a certain indication as to the strength and electrical conductivity of the weld.
This technical problem is solved by a test method having the features of claim 1. The non-destructive test method in accordance with the invention is characterized in that a defined testing force is introduced into two or more shell surface segments of the weld in directions oriented substantially towards each other or substantially to a common intersection.
The gist of the invention is based on applying a pressure to the structure of a weld of the aforementioned kind such that should a NO GO weld be involved a discernible change occurs, whereas a GO weld results in no such change and thus there is no destruction of the weld. For the first time it is now possible in accordance with the invention to make use of the xe2x80x9cuncoilingxe2x80x9d of the wires making up the strands of electrical conductors welded to each other only superficially occurring only with slight pressure loading when the weld is a NO GO.
From exhaustive tests it has been discovered that the difference between the defined testing force and the force at which a fault already occurs when the weld is a NO GO is relatively large. Thus, the force needed to test a GO weld is roughly 2.5 to 1.25 times the force needing to be applied to produce a discernible change for a NO GO weld. For example, a force of 1,000 N turned out to be sufficient as the testing force for non-destructively testing a weld made up of copper single wires having a width of 2.8 mm and a height of 2.1 mm with a ram to anvil length of 6.5 mm.
The testing force to be applied depends, of course, on the cross-sections, size and material of the single wires making up the strands of the electrical conductors as well as on the welded conditions set on the ultrasonic welding machine. However, in accordance with the invention a method of non-destructively testing such a weld is now made available for the first time which is defined by a pressure test. For this purpose it is, of course, necessary that the testing force is applied as best possible without notching the weld and that, in addition, sufficient clearance remains at the periphery of the weld so that a NO GO weld produces an uncoiling effect and thus a xe2x80x9cbloatingxe2x80x9d of the single wires for observation. This means that the nesting jaws suitable, for example, for applying the testing force in a testing apparatus do not cover the full shell circumference. Due to this it is a great advantage when the defined testing force is applied as an elongated load substantially along shell lines or narrow shell surface segments of the weld.
Due to the fact that the testing force is increased continuously from a low starting value to a defined testing force, especially in a linear increase, any commencement of a NO GO or uncoiling of the single wires in the weld is very quickly discernible. More particularly, when the force profile is plotted and analyzed any NO GO can be very quickly sensed and alerted as desired, for example, by a visual or audible indication.
As already explained, the defined testing force depends on the type of weld involved and on the conditions in ultrasonic welding. However, in general it can be said that the defined testing force is 2.5 to 1,25 times higher than the force resulting in a first discernible change in a NO GO weld.
Welds of the aforementioned kind are generally configured square or round in cross-section. Where such welds are concerned, applying the force is preferably done to two opposing longitudinal edge portions. Where oval or elliptical cross-sections are involved the force is correspondingly applied to opposing main or ancillary apexes. In the same way, where symmetrical cross-sections are involved the force is applied to advantage to opposing shell surface segments in the region of the longitudinal centerline. Otherwise, of course, applying the force to two or more longitudinal edge portions or general shell surface segments is possible and expedient.
An apparatus for implementing the aforementioned test method comprises the features as set forth in claim 11. This apparatus is characterized by two or more nesting jaws movable in directions oriented substantially towards each other or on a common intersecting direction, each of which is formed so that a weld to be tested is nested along a shell surface segment. In addition a means is needed for squeezing the nesting jaws together with a defined testing force which is so high that no disruption results with a GO weld whereas a discernible change occurs in a NO GO weld.
For this purpose the nesting jaws may be configured greatly differingly to comply with the cross-sections of the weld to be tested. For example, a wedge shape is suitable with or without a parallel arrangement of scores, notches or the like on the faces contacting the weld. However, concave or convex nesting jaws, especially of round or oval cross-sections, are also expedient.
In one preferred embodiment of the testing apparatus a means of limiting the testing force is provided to restrict the testing force capable of being applied as a maximum. The testing force is adaptable correspondingly to the nature of the weld to be tested.
In principle any NO GO condition of the weld may be discerned visually since an xe2x80x9cuncoilingxe2x80x9d occurs in such a NO GO condition, resulting in the outer shape of the test object changing. To advantage, however, a means of sensing the change, for example in the form of a strain gauge, force sensor or the like is provided with which the change occurring in the case of a NO GO weld can be sensed. This may also be, for example, a displacement sensor. It is on the basis of of an alert signal output by the sensing means that a visual and audible alert indication is given.
Applying the force is done preferably pneumatically or hydraulically assisted, but may of course also be applied manually in the case of small cross-sections or small testing forces.
By incorporating such a testing apparatus in or directly at the output of an ultrasonic welding machine for welding a plurality of strands of electrical conductors each made up of single wires into a weld of polygonal, round or oval cross-section, 100% testing can now be implemented facilitated and cost-effectively.